Migration occurs when living organisms move from one biome to another. In most cases organisms migrate to avoid local shortages of food, usually caused by winter or overpopulation. Animals may also migrate to a certain location to breed.

Animal migration is the relatively long-distance movement of individuals, usually on a seasonal basis. It is a ubiquitous phenomenon, found in all major animal groups, including birds, mammals, fish, reptiles, amphibians, insects, and crustaceans.[1] The trigger for the migration may be local climate, local availability of food, the season of the year or for mating reasons.[2] To be counted as a true migration, and not just a local dispersal or irruption, the movement of the animals should be an annual or seasonal occurrence, such as birds migrating south for the winter; wildebeest migrating annually for seasonal grazing; or a major habitat change as part of their life, such as young Atlantic salmon leaving the river of their birth when they have reached a few inches in size.[3]

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Migration can take very different forms in different species and as such, there is no simple accepted definition of migration. One of the most commonly used definitions, proposed by Kennedy[4] is

“Migratory behavior is persistent and straightened out movement effected by the animal’s own locomotory exertions or by its active embarkation upon a vehicle. It depends on some temporary inhibition of station keeping responses but promotes their eventual disinhibition and recurrence.”

Migration has also been described as a term that describes the four related concepts:[1]

persistent, straight, movement behavior

relocation of an individual on a greater scale (both spatially and temporally) than its normal daily activities

seasonal ‘to-and-fro’ movement of a population between two areas

movement leading to the redistribution of individuals within a population

Migration can be either obligate, meaning individuals must migrate, or facultative, meaning individuals can choose to migrate or not.

Within a migratory species or even within a single population, often not all individuals migrate. Complete migration is when all individuals migrate, partial migration is when some individuals migrate while others do not, and differential migration is when the difference between migratory and non-migratory individuals is based on age or sex (for example).[1]

While most migratory movements occur on an annual cycle, some daily movements are also referred to as migration. For example, many aquatic animals make a vertical migration (Diel vertical migration), travelling a few hundred metres up and down the water column.[5] Similarly, some jellyfish make daily horizontal migrations, traveling a few hundred metres across a lake.[6]

Irregular (non-cyclical) migrations such as irruptions can occur under pressure of famine, overpopulation of a locality, or some more obscure influence.[7]

Range shifts are a natural response to climate change. Species with sufficient levels of mobility may respond quickly to environmental change, with species capable of undertaking long migratory movements likely to shift ranges first (Lundy et al., 2010).

“The range of plants and animals are moving in response to recent changes in climate (Loarie 2009).” As Temperature increases, ecosystems are particularly threatened when their niche has, essentially no were else to move to. This hindrance is particularly prevalent in mountain ranges for example. The speed at which climate is changing is derived from ratio of temporal and spatial gradients of mean annual near-surface temperature.

“Mountainous biomes require the slowest velocities to keep pace with climate change. In contrast, flatter biomes, such as flooded grasslands, mangroves and deserts require much greater velocities. Overall, there is a strong correlation between topographic slope and velocity from temperature change (Loarie 2009).”

Temperatures are expected to rise more than average in higher latitudes and at higher elevations. Animals living at lower elevations could migrate to higher elevations in response to climate change as temperatures rises. Whereas animals in higher elevations will eventually ‘run out of mountain’. “Results confirmed that protected large-scale elevation gradients retain diversity by allowing species to migrate in response to climate and vegetation change. The long-recognized importance of protecting landscapes has never been greater (Moritz 2008).”

Over the past 40 years, species have been extending their ranges toward the poles and populations have been migrating, developing, or reproducing earlier in the spring than previously (Huntley 2007).

Annual breeding cycles sometimes apply to mammals, with regulating environmental effects including seasonal temperature variation and food availability. Migration patterns of a mammal may sometimes govern breeding times. The Polar Bear is an example of a mammal whose breeding locations are influenced by migration movements of this species to the seasonal Arctic pack ices. In particular the Polar Bears who breed in Wapusk National Park need to migrate to the Hudson Bay pack ice.[9]

Before the phenomenon of animal migration was understood, various folklore and erroneous explanations sprang up to account for the disappearance or sudden arrival of birds in an area. In Ancient Greece, Aristotle proposed that robins turned into redstarts when summer arrived.[10] The barnacle goose was explained in European Medieval bestiaries and manuscripts as either growing like fruit on trees, or developing from goose barnacles on pieces of driftwood.[11] Another example is the swallow, which at various times was suggested to hibernate either underwater, buried in muddy riverbanks, or in hollow trees.